1. Field of Invention
The present invention relates to a metal-oxide-semiconductor structure and corresponding method of manufacture. More particularly, the present invention relates to an air gap semiconductor structure and corresponding method of manufacture.
2. Description of Related Art
Following the rapid reduction of semiconductor line width, the accompanied increase in resistor-capacitor (RC) time delay has greatly reduced the operating speed of integrated circuits. To reduce RC time delay, methods that can lower resistance is frequently adopted. The most common trend now is to replace conventional aluminum wires by copper wires.
A second way of reducing RC time delay is to reduce capacitance between conductive wires in a multi-layer design. Conventional silicon dioxide is no longer versatile enough for this purpose because silicon dioxide has a relatively high dielectric constant. In general, low dielectric constant organic or inorganic material is used to form inter-metal dielectric layers. However, a medium having the lowest dielectric constant is air (a dielectric constant of 1). Therefore, air is an ideal dielectric medium for lowering the capacitance of a capacitor.
Although air is the best dielectric material for lowering capacitance, overall mechanical strength of the device is reduced correspondingly. A weakened structure can have serious effects in various aspects of subsequent integrated circuit fabrication.
FIG. 1 is a schematic cross-sectional view showing a conventional semiconductor device that uses air as a dielectric medium. As shown in FIG. 1, the structure includes a metallic line 102 over a substrate 100. A silicon dioxide layer 104 is attached to the upper surface of the metallic line 102, thereby forming air pockets 106 and 108 that also serve as a dielectric layer.
In the aforementioned implementation, the metallic lines 102 are usually separated from each other by a distance. Air pockets such as 108 may destabilize the semiconductor structure leading to ultimate deformation. Moreover, the air pockets 106 and 108 are normally distributed all over the substrate 100. Although capacitors have low capacitance when air is used as a dielectric medium layer, the poor heat conductivity of air often results in a rapid rise in temperature during operation.